Great question! In my last Ask-a-Scientist response, I defined a cyborg as, “a human who has certain physiological processes aided or controlled by mechanical or electronic devices.” This means that anyone who uses prosthetic limbs counts as a cyborg. I'm sure you'll agree that—with the exception of some artificial parts—most cyborgs already look pretty human! However, you're probably looking for an answer that is more than skin deep.

For a cyborg to appear as human as possible, his artificial body parts must look and move as similar to his non-artificial body parts as possible. To help understand why, let's imagine a man named Bill. Bill is a young, muscular white male who loves to play the drums. Bill also happens to be a cyborg because he has a prosthetic left arm. For Bill's left arm to look and act as “human” as possible, it must also look white, muscular, and be able to move around as smoothly as his right arm when playing the drums—otherwise something might look out of place. How do you turn a metal-and-plastic prosthesis into a human-looking arm?

Even though scientists and doctors still cannot grow real human skin over an artificial arm, they can cover it with something that looks almost as good: silicone rubber! Silicone rubber (different from the kind used in rubber bands) can look very similar to real human skin. Skilled artists can shape and color the rubber so it has the same nuances and imperfections as a real arm. This rubber is so realistic, it has even been used to make robots—which have no human parts whatsoever—look just like you and me! For example, scientists at Osaka University and the Kokoro Company in Japan have developed a humanoid robot called “Actroid” that, at a quick glance, looks just like a regular Japanese woman. Actroid is so convincing because the silicone skin covering her robotic body was crafted to look exactly like that of a real woman. However, as convincing as Actroid is, she gives herself away as a robot because her motions are powered by motors and air pumps that cannot react as naturally as real human muscle.

To overcome this challenge, engineers have developed fancily-named materials called electroactive polymers (EAPs). EAPs can be used in place of clunky pumps and noisy motors to make artificial body parts that move more like real body parts controlled by real human muscle. Our drummer friend Bill is able to move his right arm and play his drums because his arm muscles can rapidly contract and relax. EAPs can be structured in the same way as real muscle and—when a certain voltage is applied--can also rapidly contract and relax. Some EAPs are so similar to real muscle that they are called “artificial muscles”.

In the future, prosthetic arms and other synthetic additions may become so human-like that they are virtually indistinguishable from the regular old-fashioned flesh-and-blood versions. As described in my article, the same technology can be used to make entirely artificial beings, like robots, look just like humans. If robots can pass as humans, and humans with robotic body parts can pass as humans, then what does it mean to look like a human?